164 FUNGOUS DISEASES OF PLANTS 



short, so that the fungus spreads with great rapidity. Oospores 

 are commonly produced. 



Control. It would seem that this fungus has been controlled 

 in New York by systematically spraying where it is likely to be 

 abundant after June 15. In addition, however, it is important to 

 destroy the tops of diseased plants, and by no means to return 

 these to the land or throw them on the compost heap, since the 

 oospores retain their vitality a long time. Rotation of crops is 

 also important. 



XVII. PERONOSPORA: OTHER SPECIES 



Peronospora sparsa Berk, is not an uncommon parasite of 

 cultivated roses in Europe. At times it has been productive 

 of serious epidemics. Affected leaves show brown spots on the 

 upper surfaces not unlike the blotches produced by other fungi, 

 but on the under surfaces the repeatedly dichotomous conidio- 

 phores appear in sufficient quantity to be readily recognized as 

 a mildew. 



Peronospora effusa (Grev.) Rabh. develops during moist weather 

 a destructive disease of spinach (Spinacia oleracea), and it is also 

 common upon other members of the Chenopodiaceae, as well as 

 upon certain Plantaginaceae. Pale or water-soaked spots are pro- 

 duced and the leaves may be rapidly killed. Ordinarily oospores 

 are found in quantity. 



XVIII. DOWNY MILDEW OF THE LETTUCE 

 Bremia Lactuca Reg. 



Downy mildew of the lettuce is not an infrequent pest where 

 lettuce is grown under glass during the winter and spring. It 

 also occurs with cool weather in the open, particularly upon fall 

 lettuce. This fungus is also quite generally distributed on several 

 species of Senecio, Sonchus, and Lactuca as well as on a few 

 other species of Composite. Upon lettuce the conidiophores of 

 the fungus appear on the under side of the leaf, and the areas 

 affected are at first merely paler in color, afterwards wilting. 



